Ductless dryer

ABSTRACT

A ductless dryer includes a main body, a drum rotatably installed at the main body, a heat exchanger for removing moisture from air exhausted from the drum, a circulation duct to flow the air exhausted from the drum into the heat exchanger, an exhaust duct to flow the air exhausted from the heat exchanger outside the dryer; and a noise reduction portion to attenuate noise propagation through the exhaust duct. As the ductless dryer is provided with the noise reduction portion, the noise propagation through the exhaust duct exposed into the room and the noise of the entire ductless dryer can be attenuated at the same time, whereby a quieter room environment can be provided.

This application claims the benefit of the Korean Patent Application No. 10-2007-0017172, filed on Feb. 20, 2007, which is hereby incorporated by reference in its entirety as if fully set forth herein.

BACKGROUND

1. Field of the Invention

The present invention relates to a ductless dryer, and particularly, to a ductless dryer in which noise is attenuated completely by attenuating the noise transferred through an exhaust duct exposed into a room.

2. Description of the Related Art

In general, a dryer, e.g. a clothes dryer, is an apparatus performing a drying operation on objects such as wet laundry to be dried by blowing hot air generated by a heater into a drum to absorb moisture from the objects therewithin. Dryers can be categorized as exhausting (e.g. vented) type dryers and condensing (e.g. ventless) type dryers depending on the method employed for dealing with the humid air generated as the objects are dried by absorbing moisture therefrom.

In the exhausting type dryer, humid air exhausted from a drum in which the objects to be dried are held is exhausted i.e., vented outside the dryer. However, an exhaust vent or duct is required for exhausting the moisture evaporated from the objects in the drum to the outside of the dryer, and especially, in the case of a dryer heated by gas, the exhaust duct should be installed being extended a long distance to the outside of a room or building, because products of combustion such as carbon monoxide etc. are exhausted together with the moisture.

Meanwhile, in the condensing type dryer, the moisture in the humid air exhausted from the drum is condensed at a heat exchange unit to remove the moisture therefrom, and the dried air is recirculated back into the drum. However, a condensing type dryer does not facilitate to use gas as a heating source because a closed loop may be formed due to the recirculating flowing of the drying air.

In a ductless dryer, these disadvantages of the exhausting type and the condensing type dryers may be improved upon. That is, the ductless dryer can use gas as its heating source, and accordingly it can be maintained inexpensively, although it is required to have an exhaust duct installed to be extended a long distance to the outside of the room.

However, in the case of the ductless dryer, because the exhaust duct is exposed into the room, accordingly noise may emanate into the room through the exhaust duct, and thereby, the room in which such ductless dryer is situated may not be quiet.

SUMMARY

Therefore, it is an object of the present invention to provide a noise-attenuated ductless dryer. Further, it is another object of the present invention to provide ductless dryer attenuating the noise which is propagated through an exhaust duct exposed into a room.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a ductless dryer including: a main body; a drum rotatably installed at the main body; a heat exchanger for removing moisture included in air exhausted from the drum; a circulation duct to conduct the air exhausted from the drum to the heat exchanger; an exhaust duct to conduct the air exhausted from the heat exchanger outside the dryer; and a noise reduction portion to attenuate noise from being propagated through the exhaust duct.

Herein, preferably, the noise reduction portion may be installed between the circulation duct and the heat exchanger in consideration of space limitations. Further, preferably, the noise reduction portion is connected with the heat exchanger in a straight line in order for the air from the noise reduction portion to be directly introduced to the heat exchanger. Further, preferably, in the noise reduction portion, first, second and third pipes are sequentially connected, and a cross-sectional area of the second pipe is larger than the cross-sectional areas of the other pipes, the first and the third pipes.

Herein, more preferably, the first and the third pipes have the same cross-sectional area as each other.

Further, preferably, the noise loss transfer function (LT) through the noise reduction portion can be determined by the following formula:

$\begin{matrix} {L_{T} = {10{\log_{10}\left( \frac{I^{2}}{T^{2}} \right)}}} \\ {= {10{\log_{10}\left\lbrack {1 + {\frac{1}{4}\left( {\frac{A_{1}}{A_{2}} - \frac{A_{2}}{A_{1}}} \right)^{2}{\sin^{2}\left( \frac{wl}{c} \right)}}} \right\rbrack}}} \end{matrix}$ $\frac{wl}{c} = {\frac{2\pi \; {fl}}{c} = {\frac{{2n} - 1}{2}\pi}}$ $l = {\frac{1}{4}\frac{c}{f_{1}}}$

wherein, I: noise at an inlet of the second pipe T: noise at an outlet of the second pipe A1: cross-sectional areas of the first and the third pipes A2: cross-sectional area of the second pipe l: length of the second pipe c: speed of sound f: frequency n=1, 2, 3 . . . f1: target noise frequency (n=1).

Meanwhile, the noise reduction portion may include an inner pipe; and an outer pipe enclosing the inner pipe to form a noise space, and the inner pipe may be provided with a plurality of noise openings therein in communication with the noise space. Herein, preferably, the outer pipe entirely or partly encloses the inner pipe. If the outer pipe encloses only a part of the inner pipe, the enclosing angle (Θ) is 120° or less, preferably. Herein, preferably, the outer pipe includes an outer wall enclosing the inner pipe and being spaced therefrom in a radial direction; and lateral walls forming the noise space by closing front and rear ends of the outer wall. Further, preferably, the target noise frequency to be attenuated through the noise reduction portion is determined by the following formula.

$f_{res} = {5000\sqrt{\frac{p}{l\left( {t + {0.8d}} \right)}}}$

wherein, p: percentage of the area of the openings in the area of the inner pipe forming an interior surface of the space l: separation distance between the inner pipe and the outer pipe t: thickness of the inner pipe d: diameter of each opening f_(res): target noise resonant frequency

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a ductless dryer having a noise reduction portion in accordance with a first embodiment of the present invention;

FIG. 2 is a plan view showing an inside of the ductless dryer of FIG. 1;

FIG. 3 is a diagram showing an arrangement for measuring the noise of the ductless dryer in order to design the noise reduction portion in FIG. 1;

FIG. 4 is a graph showing the result of the noise measurement by the arrangement in FIG. 3;

FIG. 5 is a diagram showing an arrangement for measuring the noise transferred through an exhaust duct of the ductless dryer considered in order to design the noise reduction portion in FIG. 1;

FIG. 6 is a graph showing the result of the noise measurement by the arrangement in FIG. 5;

FIG. 7 is a diagram showing the noise reduction portion in FIG. 1;

FIG. 8 is a graph comparing the noise generated when the noise reduction portion is mounted and is not mounted;

FIG. 9 is a diagram showing a noise reduction portion in accordance with a second embodiment of the present invention;

FIG. 10 is a cross-sectional view taken along line X-X in FIG. 9;

FIG. 11 is a cross-sectional view taken along line XI-XI of FIG. 9; and

FIG. 12 is a graph comparing the results of noise measurement according to the changes in the number of noise reduction portion exhaust openings in the noise reduction portion of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, description will now be given in detail of a ductless dryer having a noise reduction portion in accordance with a first embodiment of the present invention. FIG. 1 is a schematic view of a ductless dryer having a noise reduction portion in accordance with the first embodiment of the present invention, and FIG. 2 is a plan view showing the inside of the ductless dryer of FIG. 1. The solid line arrows indicate the flowing paths of air.

Referring to FIGS. 1 and 2, the ductless dryer having the noise reduction portion in accordance with the first embodiment of the present invention includes a main body 110, a drum 120 rotatably installed at the main body 110, a hot air supplying unit 140 providing hot air into the drum 120, a heat exchange unit 150 condensing moisture included in the air exhausted from the drum 120 through heat exchange with cooler exterior air, a circulation duct 114 conducting the air exhausted from the drum 120 to the heat exchange unit 150, an exhaust duct 181 conducting the air exhausted from the heat exchange unit 150 outside the dryer, and a noise reduction portion 160 for attenuating noise so as to prevent the noise from being transferred through the exhaust duct 181. Herein, the noise reduction portion 160 may be installed between the circulation duct 114 and the heat exchange unit 150 in consideration of a limitation of space inside the dryer. Further, the noise reduction portion 160 may be connected inline with the heat exchange unit 150 in order for the air from the noise reduction portion 160 to be directly introduced to the heat exchange unit 150 with a minimized resistance.

A door 111 may be installed at the front surface of the main body 110 so as to permit the loading of clothes into the drum 120, and feet 113 are installed at the under side thereof for supporting the main body 110. A belt 131 rotating the drum 120, a fan 133 installed inside the circulation duct 114 blowing the air inside the ductless dryer and a motor 135 providing driving power to the belt 131 and the fan 133 may be installed inside the main body 110. Herein, the belt 131 and the fan 133 may each be provided with driving power, respectively, by separate motors 135. Meanwhile, a filter (not shown) may be installed at the circulation duct 114 so as to filter lint debris such as fluff or waste thread included in the hot and humid air flowing from the drum 120.

The drum 120 may be a barrel provided with an inner space so as for objects to be dried such as clothes to be loaded thereinto, in the interior of which a plurality of lifters or baffles 121 may be installed to lift the clothes for exposing them to the flow of hot air.

The hot air supplying unit 140 may include a valve 141 supplying and shutting off the supply of gas, a gas burner 143 mixing the gas admitted from the valve 141 with air provided from outside and then igniting the mixture to generate hot air and a hot air supplying duct 145 communicating the gas burner 143 with the drum 120 so as to supply the hot air into the drum 120. A flame rod (not shown) may be installed in the hot air supplying unit 140, being installed extendingly to the edge of a flame region there in order to monitor the burner flame so as for a burner control unit (not shown) to indirectly measure the emitted amount of carbon monoxide (CO) through a detected value of a current flowing in the flame rod. The burner control unit functions to regulate the operation of the valve 41, and thereby to regulate the combustion of the gas-air mixture in the burner 143, by sensing the current flowing in the flame rod, and upon sensing a flame rod current valve signifying interrupted or abnormal combustion, the burner control unit operates to cause immediate closure of valve 141 in a known manner.

Preferably, a solenoid valve is used for the valve 141 so as to enable the burner control unit to sensitively adjust the supplied amount of gas.

The gas burner 143 may be connected with the valve 141 to heat air with heat generated by burning a mixture of the gas supplied from the valve 141 and outside air. And then, the hot air is supplied to the drum 120 through the hot air supplying duct 145.

The heat exchanger 150 is composed of fins 151 and tubes 153, and makes the hot and humid air from the drum 120 dry by condensing the moisture from the air with low-temperature water using a method of heat exchange between the air and water. An inlet of the heat exchanger 150 is connected with the drum 120 by the circulation duct 114, and an outlet thereof is connected with the exhaust duct 181.

The fins 151 may each be formed of a sheet of a metal material having an excellent thermal conductivity, and a plurality of such sheets are stacked at fine intervals adjacent each other in order for hot and humid air to pass therethrough while contacting with the sheets.

The tubes 153 may be provided with low-temperature (22° C.) water circulating therein, and penetrate the fins 151 in a back-and-forth serpentine manner. Water lines (not shown) for supplying and returning the low-temperature water are connected to both ends of the tubes 153. A water receiver (not shown) for collecting condensed water which is generated in the condensing process and falls from the fins may be installed at the lower portion of the heat exchanger 150.

FIG. 3 is a diagram schematically showing an arrangement for measuring the noise of the ductless dryer in order to design the noise reduction portion in FIG. 1, FIG. 4 is a graph showing the result of the noise level measurement in FIG. 3, FIG. 5 is a diagram schematically showing an arrangement for measuring noise transferred through an exhaust duct of the ductless dryer in order to design the noise reduction portion in FIG. 1, FIG. 6 is a graph showing the result of the noise measurement in FIG. 5, FIG. 7 is a diagram showing the noise reduction portion in FIG. 1, and FIG. 8 is a graph comparing the noise levels generated when the noise reduction portion is mounted and is not mounted. The bold line marker indicates the movement of the noise level.

Referring to FIG. 3, the noise level of the ductless dryer may be measured by setting a microphone at a position 1 m away from the rear surface of the ductless dryer. Referring to FIG. 4, as a result of the measurement, the noise having a component near 250 Hz (circle-enclosed portion) may be the greatest.

Referring to FIG. 5, the noise emitted through the exhaust duct 181 may be measured by setting the microphone at a position 0.1 m away from the exhaust duct 181. Referring to FIG. 6, as a result of the measurement, the noise emitted through the exhaust duct 181 may be the greatest at a component near 250 Hz (circle-enclosed portion).

Referring to FIGS. 4 and 6, the frequency component of the noise of ductless dryer and that of the noise propagating through the exhaust duct 181 are almost similar; thus, by attenuating the noise propagated through the exhaust duct 181, the noise of the ductless dryer can be also attenuated. That is, in conclusion, to attenuate the noise of the ductless dryer and the noise emitted through the exhaust duct 181 at the same time, the noise having a 250 Hz component propagating through the exhaust duct 181 should be attenuated.

Referring to FIG. 7, the noise reduction portion 160 designed according to the above objective is composed of a first pipe 161, a second pipe 163 and a third pipe 165, which are sequentially connected, and a cross-sectional area of the second pipe 165 (A2) may be larger than those of the first pipe 161 and the third pipe 165. And, the cross-sectional areas of the first pipe 161 (A1) and the third pipe 165 (A3) are the same. As the noise reduction portion 160 is designed to have different cross-sectional areas of the pipes, the acoustic impedance may be different, and thereby sound with a frequency component according to characteristics of its wavelength can not pass through smoothly.

Meanwhile, the noise loss transfer function (LT) of the noise reduction portion (16) is determined by the following formulas. The larger the noise loss transfer function (LT) is, the less the noise propagated through the exhaust duct (181) is.

$\begin{matrix} \begin{matrix} {L_{T} = {10{\log_{10}\left( \frac{I^{2}}{T^{2}} \right)}}} \\ {= {10{\log_{10}\left\lbrack {1 + {\frac{1}{4}\left( {\frac{A_{1}}{A_{2}} - \frac{A_{2}}{A_{1}}} \right)^{2}{\sin^{2}\left( \frac{wl}{c} \right)}}} \right\rbrack}}} \end{matrix} & \left\lbrack {{first}\mspace{14mu} {formula}} \right\rbrack \\ {{\frac{wl}{c} = {\frac{2\pi \; {fl}}{c} = {\frac{{2n} - 1}{2}\pi}}}{l = {\frac{1}{4}\frac{c}{f_{1}}}}} & \left\lbrack {{second}\mspace{20mu} {formula}} \right\rbrack \end{matrix}$

wherein, I: noise level at the inlet of the second pipe T: noise level at the outlet of the second pipe A1: cross-sectional areas of the first and the third pipes A2: cross-sectional area of the second pipe l: length of the second pipe c: speed of sound f: frequency n=1, 2, 3 . . . f1: target noise frequency (n=1).

If the target noise frequency f1 to be attenuated in accordance with the formulas above is set to 250 Hz, the length l of the second pipe 163 is calculated as 334.6 mm by the second formula. In addition, considering an installed position in the ductless dryer and the size of the noise reduction portion 160, when the cross-sectional areas A1, A2: are adjusted to appropriated sizes to calculate the maximum value of the noise loss transfer (LT) by the first formula, the diameter of the first pipe 161 and the third pipe 165 (d1) is 100 mm, and the diameter of the second pipe 163 is 140 mm.

In other words, when the target noise frequency f1 is set to 250 Hz, the length l of the second pipe 163 is 334.6 mm, the diameter of the first pipe 161 and the third pipe 165 (d1) is 100 mm, and the diameter of the second pipe 163 is 140 mm.

Referring to FIG. 8, when the noise reduction portion 160 designed as described above is mounted at the ductless dryer, the noise generated near 250 Hz is remarkably attenuated by 10 dB or more.

In conclusion, by employing the noise reduction portion 160, the noise propagating through the exhaust duct 181 exposed into the room and the noise of the entire ductless dryer are attenuated at the same time, and thereby, a quieter room environment can be implemented.

FIG. 9 is a perspective schematic diagram showing a noise reduction portion in accordance with a second embodiment of the present invention and a noise level test setup therefor, FIG. 10 is a diagram showing a cross-section X-X of the noise reduction portion in FIG. 9, FIG. 11 is a diagram showing a cross-section XI-XI of the noise reduction portion in FIG. 9, and FIG. 12 is a graph comparing results of noise level measurements according to varying the number of noise reduction portion exhaust openings in the noise reduction portion in FIG. 9.

Referring to FIGS. 9 and 10, the noise reduction portion 170 in accordance with the second embodiment of the present invention includes an inner pipe 171 and an outer pipe 173 forming a space S by enclosing a part of or the entire inner pipe 171. A plurality of openings 171 a in communication with the space S are formed in the surface of the inner pipe 171.

Preferably, the outer pipe 173 encloses the entirety or a part of the inner pipe 171 so as to enhance the noise attenuation effect. Preferably, when the outer pipe 173 encloses only a part of the inner-pipe 171 as the second embodiment, the enclosing angle (θ) is 120° or less. In order to implement the aforementioned, the outer pipe 173 includes an outer wall 173 a enclosing the inner pipe 171 while being spaced therefrom in a radial direction; and end walls 173 b forming the airtight space S by closing the front and the rear ends of the outer wall 173 a.

Meanwhile, the target noise frequency to be attenuated through the noise reduction portion 170 as aforementioned is determined by the following third formula:

$\begin{matrix} {f_{res} = {5000\sqrt{\frac{p}{l\left( {t + {0.8d}} \right)}}}} & \left\lbrack {{Third}\mspace{14mu} {formula}} \right\rbrack \end{matrix}$

wherein, p: percent of the area of the openings in the surface area of the inner pipe forming the interior of the space l: separation distance between the inner pipe and the outer pipe t: thickness of the inner pipe d: diameter of the openings f_(res): target noise resonant frequency.

By employing the above third formula, when the target noise resonant frequency f_(res) to be attenuated is set to 254 Hz, and considering the installed position in the ductless dryer and the size of the noise reduction portion 170, when the setting of l is 50 mm, t is 3 mm and d is 7 mm, p is 1.1%.

Alternatively, when the target noise resonant frequency f_(res) to be attenuated by implementing the third formula is set to 179 Hz, and considering the installed position in the ductless dryer and the size of the noise reduction portion 170, when the setting of l is 50 mm, t is 3 mm and d is 7 mm, p is 0.55%.

Herein, it is required to form the openings 171 a more densely when the target noise resonant frequency is set to 254 Hz, comparing with the case of 179 Hz, in the surface of the inner pipe 171. Of course, it is possible to adjust variables l, d, t in consideration of the number of the openings 171 a, in a state that the target noise resonant frequency is set.

Referring to FIGS. 9 and 12, when a speaker set at the inlet of the noise reduction portion 170 generates a swept sinusoidal signal of 2 Hz˜1200 Hz, the sound level with respect to the swept sinusoidal signal was measured at the outlet of the noise reduction portion 170 by using a microphone, and the result was as follows.

In case of the noise reduction portion 170 configured for the target noise resonant frequency F254 (f_(res) of 254 Hz), l of 50 mm, t of 3 mm, d of 7 mm and p of 1.1%, the sound is largely attenuated near 254 Hz. And, in the case of the noise reduction portion 170 configured for the target noise resonant frequency F179 (f_(res) of 179 Hz), l of 50 mm, t of 3 mm, d of 7 mm and p of 0.55%, the sound was largely attenuated near 179 Hz. In other words, the openings 171 a prevent the target noise frequency component from being propagated by absorbing (trapping) sound waves of the set target noise frequency component by functioning as a Helmholtz resonator.

In conclusion, by employing the noise reduction portion 170, the noise propagation through the exhaust duct 181 exposed into the room and the noise level of the entire ductless dryer are attenuated at the same time, and thereby, the quiet room circumstances can be implemented.

Hereinafter, the operation of the ductless dryer in accordance with the first embodiment will be described.

Referring to FIGS. 1 and 2, gas supplied from the valve 141 is mixed with external air in the gas burner 143, and then the mixture is ignited to generate hot air. The generated hot air is supplied into the drum 120 through the hot air supply duct 145. The objects to be dried such as clothes in the drum 120 are dried by the supplying of the hot air. The hot and humid air obtained by drying the objects passes through the circulation duct 114 to exhaust the hot and humid air. Lint such as fluff or waste thread contained in the hot and humid air exhausted from the drum 11 passes through the filter (not shown) installed on the circulation duct 114 to be collected and removed, and the hot and humid air from which the lint has been removed is forcedly circulated.

The hot and humid air of which the noise is attenuated at the noise reduction portion 160 is condensed by contacting and passing through the fins 151 of the heat exchanger 150 to be dried, and the condensed water generated in the condensing process is collected in the water receiver (not shown). The dried air exhausted from the heat exchanger 150 is discharged into the room after passing through the exhaust duct 181.

In the ductless dryer employing a noise reduction portion in accordance with the present invention as described above, by the noise reduction portion, the noise propagation through the exhaust duct exposed into the room and the noise of the entire ductless dryer are attenuated at the same time, and thereby, quiet room circumstances can be achieved.

The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teachings can be readily applied to other types of apparatuses. This description is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. The features, structures, methods, and other characteristics of the exemplary embodiments described herein may be combined in various ways to obtain additional and/or alternative exemplary embodiments.

As the present features may be embodied in several forms without departing from the characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the appended claims. 

1. A ductless dryer, comprising: a main body; a drum rotatably installed at the main body; a heat exchanger for removing moisture included in air which has been exhausted from the drum; a circulation duct communicating the air exhausted from the drum into the heat exchanger; an exhaust duct communicating the air exhausted from the heat exchanger outside the dryer; and a noise reduction portion attenuating noise propagation through the exhaust duct.
 2. The ductless dryer of claim 1, wherein the noise reduction portion is installed between the circulation duct and the heat exchanger.
 3. The ductless dryer of claim 2, wherein the noise reduction portion is connected in line with the heat exchanger.
 4. The ductless dryer of claim 1, wherein the noise reduction portion has first, second and third pipes which are sequentially connected, and a cross-sectional area of the second pipe is larger than the cross-sectional areas of the first and the third pipes.
 5. The ductless dryer of the claim 4, wherein the first and the third pipes have the same cross-sectional area as each other.
 6. The ductless dryer of claim 5, wherein a noise loss transfer function (LT) of the noise reduction portion is determined by the following formula: $\begin{matrix} {L_{T} = {10\; {\log_{10}\left( \frac{I^{2}}{T^{2}} \right)}}} \\ {= {10\; {\log_{10}\left\lbrack {1 + {\frac{1}{4}\left( {\frac{A_{1}}{A_{2}} - \frac{A_{2}}{A_{1}}} \right)^{2}{\sin^{2}\left( \frac{wl}{c} \right)}}} \right\rbrack}}} \end{matrix}$ $\frac{wl}{c} = {\frac{2\; \pi \; {fl}}{c} = {\frac{{2\; n} - 1}{2}\pi}}$ $l = {\frac{1}{4}\frac{c}{f_{1}}}$ wherein, I: noise level at an inlet of the second pipe T: noise level at an outlet of the second pipe A1: cross-sectional areas of the first and the third pipes A2: a cross-sectional area of the second pipe l: length of the second pipe c: speed of sound f: frequency n=1, 2, 3 . . . f1: target noise frequency (n=1).
 7. The ductless dryer of claim 1, wherein the noise reduction portion comprises: an inner pipe; and an outer pipe enclosing at least a part of the inner pipe to form a space therebetween, wherein the inner pipe is provided with a plurality of openings therein in communication with the space.
 8. The ductless dryer of claim 7, wherein the outer pipe entirely encloses the inner pipe.
 9. The ductless dryer of claim 7, wherein an enclosing angle (θ) of the inner pipe by the outer pipe is 120° or less.
 10. The ductless dryer of claim 7, wherein the outer pipe comprises: an outer wall enclosing the inner pipe and being spaced therefrom in a radial direction; and end walls forming the space by closing front and rear ends of the outer wall.
 11. The ductless dryer of claim 7, wherein a target noise resonant frequency to be attenuated through the noise reduction portion is determined by the following $f_{res} = {5000\sqrt{\frac{p}{l\left( {t + {0.8\; d}} \right)}}}$ formula: wherein, p: percentage of the area of the openings in the area of the inner pipe forming the interior of the space l: separation distance between the inner pipe and the outer pipe t: thickness of the inner pipe d: diameter of the noise opening f_(res): target noise resonant frequency.
 12. A ductless dryer having an exhaust duct, comprising a noise reduction portion attenuating noise propagation through the exhaust duct.
 13. The ductless dryer of claim 12, wherein the noise reduction portion is connected in line with a heat exchanger of the dryer.
 14. The ductless dryer of claim 12, wherein the noise reduction portion has first, second and third pipes which are sequentially connected, and a cross-sectional area of the second pipe is larger than the cross-sectional areas of the first and third pipes.
 15. The ductless dryer of claim 14, wherein the first and the third pipes have the same cross-sectional area with each other.
 16. The ductless dryer of claim 12, wherein the noise reduction portion comprises: an inner pipe; and an outer pipe enclosing at least a portion of the inner pipe to form a space therebetween, wherein the inner pipe is provided with a plurality of noise openings thereon in communication with the noise space.
 17. The ductless dryer of claim 16, wherein the outer pipe entirely encloses the inner pipe.
 18. The ductless dryer of claim 17, wherein an enclosing angle (θ) of the outer pipe with respect to the inner pipe is 120° or less.
 19. The ductless dryer of claim 16, wherein the outer pipe comprises: an outer wall enclosing the inner pipe and being spaced therefrom in a radial direction; and end walls forming the space by closing front and rear ends of the outer wall. 